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If you have a "highly variable parasitic drain" then a battery isn't going to solve the problem.....INCLUDING the 266VDC Li-Ion battery pack that you're trying to tap into!

You NEED to find the source of your current leak.

Usually they fall into some predictable categories, listed in order of probability:

1. Accessory inspired wiring mod
2. Accident - improperly repaired.
3. Manufacturing defect.
4. Manufacturer design flaw.

G5s are three years old - and I'm not seeing many people complaining about having to jump dead batteries in these cars over and over again. They DO have under-sized batteries because of corporate greed - but they ARE generally sufficient to stay in service for 3-5 years.
You've 'likely' already engaged your local Toyota dealer about this since the car is only 2 years old - and even Toyota's sad warranty will cover a wiring issue in a vehicle less than two years old - unless you've already boltered through the mileage limit...

You can find some kid to manufacture a DC-DC circuit to trickle charge your AUX (13.5VDC) battery from your Traction (266vdc)...easy peasy!
HOWEVER (comma!!!!)
Such a modification would immediately and justifiably void your vehicle's warranty, AND for a pretty good reason!!!!!
You've PROVEN yourself that if you have a "highly variable parasitic drain" that draining TWO batteries with the "highly variable parasitic drain" is MUCH MORE than twice as dumb as ONLY draining the 13.5VDC battery.

Advice:
1. Fix your car.
2. Get a jump pack, and move on with your life.

 

lots of people have had 12 volt problems with the g5, and there are many threads here

 

For those not wanting to dumpster-dive into that thread - Priuses have two batteries, one being a 12v 'aux' battery and the other being the traction battery with a much larger capacity and nominal voltage.
GENERALLY speaking you do not cross connect these for pretty obvious reasons - especially in a car that becomes a 3200# piece of lawn art if the traction battery dies.

In short...(pun nearly unintended) you can safely jump start a Prius with a device that fits in your pocket.
If the traction battery is flatlined, probably the best and DEFINATELY the safest outcome will require a tow truck.

 

but not for people without electric coolers, correct?

 

so you're the last one with 12 volt problems in gen 5?

 

He is the first, the only one, and probably the last one with the particular problem he is having.

 

Always-on security systems cause similar problems. Basic tenet: thinking of using the 12 volt for anything extra? Don't.

 

Background: He installed an auxiliary 12-V LFP battery to run a cooler and connected it to the primary 12-V flooded-cell lead–acid battery using a battery insulator. Originally, the LFP battery was draining. To remedy that situation, he later got a new battery isolator with adjustable voltages and set the cutoff/cut-in voltages to the point that now, the car no longer charges the primary 12-V lead–acid battery. It drains everyday as a result.

No one else has the problem you are having. It is a problem you created yourself. Then, you blame Toyota and everyone else for your problem.

You bought one of the most advanced/sophisticated cars in the world, and you redneck-engineered it with minimal understanding of electronics if any. It is a bad combination. Things will progressively get worse. Once the warranty expires, you will be in trouble.

Even if Toyota did what you asked for, it still wouldn’t work because the car’s 12-V battery management system (BMS) doesn’t/can’t realize that the 12-V battery is constantly draining with the way you adjusted your battery isolator.

As I said a long time ago, these battery isolators are meant to be used with cars with dumb alternator charging systems that always keep the charging voltage at 14.3 V, and they will not work with cars with a 12-V battery management system (BMS), like the Gen 4/Gen 5 Prius HEV/PHEV:

VGate Battery Assistant battery monitor | Page 2 | PriusChat

Now, your second 12-V battery, which you got under warranty, is also ruined thanks to the constant full drains and you need a third one. I wonder what the dealer will think when they see your battery-isolator setup.

 

Man....
Glad I bought a GMC......

 

It is the same. GM also uses a 12-V battery management system (BMS). If you want a car with a dumb alternator/IC regulator charging system that always keeps the charging voltage at 14.3 V, you need to buy an old car.

https://www.silveradosierra.com/threads/what-was-gm-thinking.744094/

 

As long as a custom solution is needed
Antigravity Lithium battery with Restart (built in jump start with a remote)

 

My current 4 GMCs are an 06, 12, 20, and 23.
My three before that were also GMC.'s
The last two are still using their OEM batteries and I've only had a battery fail in service on the 2012 - after 6 or 7 years.
Since I always carry jump packs I didn't require any assistance with getting it replaced with the battery that's still installed.

I only have 150,000 miles of stick time (more or less) in Priuses because my company had a dalliance with hybrids in the mid 2020s that they now have gotten over.

 

there were a lot of others, but they all seem solved by a software update or new battery.